A charging system on a motorcycle which is modern and working as intended will produce voltage at the battery terminals between 13.5V DC and 14.7V DC over the entire rev range. Some European models can read up to 14.9V. At a basic level you can check this will a multimeter when you start the bike. You can also see the voltage drop while cranking which can also give insight to how strong the battery is. If the voltage starts out around 13v standing then drops below 9V when starting, it may indicate a worn battery or too much load pull from another source.
If you have connected Ring terminal connectors tor your battery so you can plug to your charger, you can simply probe the neg/pos side of the pigtail lead save taking the seat off. If a quick glance yields an issue, its time to dig deeper.
This is the first thing to check on your bike if your battery has expired suddenly, for example you rode it daily and had no previous signs of depletion, or you had it on a maintenance charger but now it has failed.
There are generally two things that happen, the battery is depleted of charge because it is not getting a charge correctly or at all, or you will have a dry and or swollen battery (in the case of a non- spillable battery) due to being overcharged.
When you find that there is a problem, keep in mind that the number 1 problem with any charging system are bad connections.
This is common especially if you have been in a wet environment like off road riding or riding in consistent inclement weather. If you are riding in boat weather, expect boat maintenance issues.
Any connection in the entire system could be a culprit. It's advisable to take off body panels, the fuel tank and seat, so you can get access to the wiring harness.
One by one, disconnect all connectors you can find. You can use a contact cleaner spray to blow them out before refitting the connector. I don’t recommend blowing them with a compressor as this can force moisture further into components. Contact cleaner is the go. Clean your battery compartment. Never ever spray contact cleaner on a lithium battery to clean it. This can create a hazard. Instead remove the battery and place it on a suitable charger and clean out the battery box with warm soapy water then dry with paper towel. Ensure battery terminals are tight and free from corrosion or damage.
Ensure all the connectors are securely in place, if they get corrosion inside the connectors, it can be hard to see, so better to pop the connector and refit.
Disconnect them one at a time, this way you won’t get lost and reconnect something the wrong way. Wires are color coded, so mistakes when refitting the connectors are almost impossible. If you are working with some backyarder nightmare, color blind or not going to finish in one sitting, you can stick a little tape on each wire and number match them.
There is an excellent fault finding chart available for download here https://cdn.shopify.com/s/files/1/1091/5694/files/fault-finding-diagram.pdf?235929069374954073
Go through the tests step by step, following the procedure accurately. If you're not completely sure that you have the knowledge required with electrical systems on motorcycles, find someone experienced at a local mechanic where most have testing tools available.
What is in a Powersport Charging System?
Basically, a stator and a regulator rectifier.
The electrical system on all roadgoing vehicles includes a way to generate, store, and appropriate electricity to run the machines systems. A stator generates power by producing AC (alternating current). Alternating current is often used exclusively to power halogen bulbs, tail lights, and modern ignition systems on mopeds, dirt bikes, and ATVs. Powersports vehicles that have a starter motor need to also have a battery for powering it.
Since a battery cannot store AC current, the current must be converted into a storable form of power known as DC (direct current). The regulator/rectifier enables the conversion from AC to DC current. The other job of the regulator is to keep the power level (voltage) from going above the required setting 13.8 -14.5 volts needed to power and maintain a 12V battery. If the voltage goes higher, it will cause the battery to gas excessively and become overcharged. This is where people can also run into trouble using Lithium batteries since often the charge rate is not tightly controlled at 14.4v maximum thus the charging systems that often put out higher voltages are not designed for Lithium batteries and can create dangerous overcharge situations. Lead acid batteries on the other hand are quite inert, and will most likely give off a rotten egg smell when they are on their last gasp. The machine will eventually stop.
The role of the stator is to create more power than required by the vehicle so that the battery will recharge then remain optimally charged. If the vehicle, or any additional accessories you have added to it use more power than the stator can provide, the battery will start to drain. This will typically happen if the voltage drops below 13 volts.
A traditional alternator makes AC power by using the DC power from a battery source. The benefit to this system is that it produces more power at lower engine speeds. Cars, trucks, tractors and most vehicles use this system. The key to this system is that it requires a charged battery to operate. Powersport vehicles often require a lighter weight system that can produce power with no battery at all, or with a small battery due to confined space. In fact, most batteries fitted to powersport vehicles are at least half the capacity or less than an average car when related to engine size. This requires a slightly different way of making power.
The stator on a motorcycle or ATV converts kinetic (motive) energy into electricity to charge the battery. There are two parts; the stator and the magnet rotor, also known as the flywheel. The rotor has permanent magnets and spins around the stator, this creates electric current.
Stators differ depending on requirements of that model. Some stators produce AC power for the ignition system only while others produce AC which is then converted to DC for powering lighting, ignition computers, fuel injection, etc.
Both stators and regulators can have issues from heat sink, poor placement, dirty oil, poor connections and degradation over time. Regulators tend to suffer faults when they get hot, which can show as ok when the vehicle is cold and at idle, only to fault when it gets hot or higher in the rev range. Therefore, the only real way to test these components is to run them on a dyno and all speeds for at least 10 minutes while observing the charge rates.
They can create undercharge or overcharge situations. When undercharging you will see weak voltage at the terminals with engine running, in overcharging you will see voltage well in excess of the set 13-14.7v.
There are widely available testers that will immediately pick up on more obvious issues.
Summary
It is important to understand that in the entire charging system the battery is always the weak link or a fuse. Any issue with the charging system will generally show up at the battery first. Therefore, consumers will automatically think the battery is at fault. In reality this is rarely the case since battery manufacture is a repeatable process of molded parts which are assembled in a case.
If the battery works from the outset, it likely will continue to work well so long as its charge is always optimized and it is not left sitting with depleted voltage, or had its voltage drawn down on. Both these things will destroy a battery well before its time.
The Key is to always plug your maintainer in after every ride to keep your battery in optimized health. Every time you are not doing that you are spending stored energy capacity in advance by risking sulphation and degradation to the cell plates. This is costing you money and reliability.